Population: The grey squirrel was introduced to Great Britain in the mid-19th century and after many releases it began to increase dramatically at the beginning of the 20th century, mainly spreading from Woburn Park, Bedfordshire. They came to England from North America and are now one of Britain's most well-known and frequently seen mammals, with an estimated population of 2 million. They are much more common than the native red squirrel, which has a population estimated at 161,000.